tools: split filesystem into read, write and file_ai ...

- filesystem_read: read, list, find, find_up, grep, info, exists, diff
- filesystem_write: write, append, edit, edit_lines, insert, mkdir, move, copy, delete
- file_ai: ask (was query), edit (was smart_edit)
- paged numbered reads (500 lines / 50 KB), raw copy mode, binary refusal
- nearest-match and numbered-copy hints on failed edits, context after edit
- guards: empty overwrite, non-empty delete, move/copy overwrite
- cross-tool redirects instead of silent wrong-action errors
- legacy 'filesystem' stays for archived sessions (compression, renderers, webclient)
- profiles, prompts, docs and manuals updated; TUI and webclient render the new tools

tools: split the filesystem tool into a package ...

`tools/filesystem.py` was 1145 lines: path/format/diff/AI-text helpers at the
top, then a `FilesystemTool` class with every action handler. Move the pure
layer out into `navi/tools/_filesystem/`:

* `helpers.py` — path checks, formatting, line numbering, unified-diff
  annotation, chunking, edit-op validation/application, AI prompts;
* `tool.py` — the dispatcher and its read/write/AI handlers.

The package is underscore-prefixed because `navi/tools/filesystem` also has
to stay available as the compatibility shim at the same import path. No
behavior change: the shim re-exports every top-level name the module had, and
the tests patch `settings` on the modules that now read it.